This invention pertains to the art of induction heating and more particularly to a method and apparatus for alloying a coating to an item with induction heating techniques.
The invention is particularly applicable to a method and apparatus for applying a zinc coating to a strip steel product that is typically employed in the manufacture of automobiles. However, it will be appreciated to those skilled in the art that the invention could be readily adapted for use in other areas of application as, for example, where other coatings are employed to protect other types of items.
The process of galvanizing, or the coating of iron or steel products with rust-resistant zinc, is well known. Galvannealing is also well known and provides the advantage of producing a coating which is comprised of iron-zinc alloy phases, in contrast to the essentially pure zinc coating provided on galvanized steel. A galvannealed product has a number of advantages over a galvanized product and in particular is easier to spot weld and has better paintability. Hence, galvannealed steel is more valuable to the automobile manufacturing industry.
The galvannealing process essentially consists of submerging a clean, preheated steel strip in a bath of liquid zinc. As the strip emerges from the bath, it passes through a coating control system, such as an air knife, which is used to control the thickness of the coating. The coated strip is then reheated in a galvannealing furnace to produce further intermetallic diffussion between the zinc and the steel substrate. A galvannealing furnace is typically fuel fired, but it is also known that the heating may be done electrically by induction heating coils. After exiting from the furnace the strip is allowed to cool.
There are at least two types of galvannealed products: one has equal coating on either surface of the strip, the other is described as "AB product" and has a different coating weight on each surface controlled by adjustment of the air knife. In an AB product essentially all the free zinc is removed from one surface while the other surface has various coating weights remaining.
A particular problem which has been encountered in the industry with the galvannealing installations that employ induction heating coils is the occurrence of lines or stripes in the coated steel product. The stripes appearing in the finished product are typically parallel to the direction of movement of the strip through the furnace. Occurrence of the stripes is coincident with an audible noise so that the characteristic has been referred to as "noise stripes." The stripes are actually evidence of a resonant response in the strip to the induction heating field of the induction heating coils. It has been experimentally established that as the temperature of the coated steel strip rises in the furnace, its changing characteristics may at least somewhere along the length of the heating zone produce the proper conditions for resonant vibration in the strip. This resonant vibration causes the formation of stripes in the work product. The vibration results in a non-uniform surface in the finished product making it unacceptable as a high quality commercial product.
The galvannealing process requires precise control of many variables to produce a satisfactory product. Some of which are strip temperature, speed and tension; bath temperature and composition; coating control; heating; and cooling.
When the heating was performed by induction heating techniques an additional variable was added, operating frequency. Even though induction heating provided improved control over the heating, it disappointingly also caused the noise stripe problem. Surprisingly, mere tuning or adjustment of the operating frequency failed to avoid the noise line problem. Even more surprisingly, further adjustment of the other variables also failed to eliminate the problem.
The noise stripes have been more technically identified as resulting from transverse flexural resonance in the steel strip. The term originates from a vibrational flexing in the strip transverse to its direction of movement set up by an occurrence of the resonant vibration in the strip.
When the inventors identified the problem as transverse flexural resonance in the strip, they realized that mere adjustment of the operating frequency could not avoid the problem because the continuously varying characteristics of the strip through the heating zone could establish the conditions so that resonance would be satisfied at some point.
The present invention contemplates a new and improved method and apparatus which overcomes the above-referred to problems and others to provide a new method and apparatus which is simple in design, readily adaptable to a plurality of uses with a variety of items having a variety of dimensional characteristics, and which precludes the production of noise stripes in a galvannealed product.